The present invention relates generally to the field of monitoring travelling products, and more specifically, to the field of detecting edge cracks on continuous paper webs.
It is well known that during the manufacture of continuous, travelling products, such as paper webs, cracks in the edges of the products often appear. Besides reducing the level of quality of the finished product, unattended cracks often lead to breaks in the product during high tension stages of the process. The full extent of the harmful consequences of a break in a high speed process is also well known in the industry. The large amount of time it takes to repeatedly stop the process, clean-up the results of the break, and re-start the process translates into a very large amount of cost to a manufacturer.
The early detection of an edge crack in a travelling product is desirable because it can enable a manufacturer to appropriately react to such a crack. One type of reaction involves easing the tension normally applied to the product during high tension stages of the process while the cracked portion travels through that stage. In some cases, a simple slowing of the process may adequately relieve the tension. Another type of reaction involves selectively trimming the edge of the product around the crack. A smooth, gradual cut in the product is known to be much stronger than an edge crack.
When faced with the costs related to frequent breaks and the problem of reliably detecting edge cracks, some manufactures routinely trim away large continuous portions along the edge of the product in an effort to eliminate all of the area susceptible to cracks before the product reaches the high tension stages of the process. This procedure is obviously very wasteful and is also unreliable, given known limitations of common trimmers.
There have been some attempts to design comprehensive fault detection systems to detect most types of faults located across the entire width of the product. Most of these systems are extremely expensive, involve a multitude of elements, and are difficult to operate and maintain. Although many of these systems claim to be able to detect edge cracks, all have limitations which characterize the existing problems in the art of detecting edge cracks.
There are at least two major types of edge cracks which are frequently encountered: separated & non-separated. Separated edge cracks are those edge cracks which are readily seen from above as gaps. Typically, at least one portion of the product adjacent to the crack is folded back over itself to reveal a large, sharply angled hole. A few of the current fault detection systems are relatively effective in detecting these types of separated edge cracks where the crack is visible from directly above the crack. The second major type of edge crack, the non-separated edge crack, is not readily seen from above as a gap. With this type of crack, both portions of the product adjacent to the crack are so close to each other that there is no gap readily visible from directly above the crack. There is currently a great need in the industry for a system which will effectively detect non-separated edge cracks.
Of the types of systems which are relatively effective at detecting separated edge cracks only, a first type of system utilizes one or more cameras positioned directly above the travelling product. The cameras continually take segmented pictures of the product as it travels directly below. The pictures are broken down into very small segments and analyzed for various types of faults. This analysis process normally requires a high-powered, and consequently high-priced, computing device to continuously process the very large amount of data. Because some travelling products travel very quickly, the camera systems are required to scan very rapidly to be able to detect the smaller faults.
Assuming the scan rate can be adjusted to accommodate both the speed of the travelling product and the size of a separated edge crack, these camera systems may be used to detect separated edge cracks. By comparing successive positions of the edges between scans, a crack is assumed to have been found when the values differ greatly. That is, when the edge wanders in very rapidly, a crack signal is generated. This method, when considered in light of the complicated construction, the high cost, and the relative slowness, is not very efficient at detecting separated edge cracks. Furthermore, because non-separated edge cracks often appear from above as continuous sections of product, this type of system is virtually ineffective at detecting the common non-separated edge cracks.
A second type of system utilizes one or more rapidly scanning lasers. These systems deploy rapidly moving parts, such as mirrors, which direct one or more laser beams directly down onto the product and transversely across the width of the travelling product. Besides having problems associated with these continuously moving parts, the problem of missing a crack between scans is also readily apparent. During the time in which the laser is scanning other parts of the product, a very thin crack in a rapidly travelling product may pass the point at which the laser scans. Furthermore, problems similar to those associated with the camera systems often exist with these laser systems too.
A third type of system involves a series of phototransistors positioned above the product in a line across the width of the product. Light is supplied to the underside of the product by a continuous light source positioned below the series of phototransistors. The intensity of the light transmitted through the product is monitored and used to detect faults. Because the edges of many travelling products wander, screens are routinely placed over a predetermined number of phototransistors to block their reception of light and eliminate false detections due to edge wander. However, this shielding process also blocks the system's ability to detect separated edge cracks on products with wandering edges. Finally, as with the other systems, this type of system is also completely ineffective at detecting non-separated edge cracks.
There is, therefore, a need in the art to provide a method and apparatus for detecting both separated and non-separated edge cracks on various types of travelling products without encountering these and other types of problems.